It will drop your speed a knot or two--or the other way to say, about 5% to 10% more fuel. A lot depends on the "Boat" A small Livingston will be lighter than the Whaler--I would not tow a Squall, and an 11 or 13 will make a lot more fuel difference than the Livingston. My concern is the possible swamping in rough weather if towing fast. For us an inflatable is far more practical--although we have had both hard dinghies and RIB's on our larger boats--again we brought them aboard with davits or a roller system when under way 99% of the time.
If you are towing at low speed--then far more practical. I towed a 12' aluminum Duraboat behind our Tom Cat and 25 C Dory for relatively short distances--it skated all over the surface of the water when on a plane. With bigger boats I have towed an 20 foot Grady White--but only at 8 knots.
There are several trawler owners who tow Livingstons--and they don't consider them good boats for long distances (an exception might be the larger 14 footers)--The "BullFrog" is a popular boat to tow.
Consider what you are going to do with a large heavy dinghy which is towed if you come to a dock.
